1. Field of the Invention
The present invention relates generally to ink-jet printing and, more specifically, to microstepping the print media between printing passes in ink-jet hard copy apparatus having printheads firing the same colorant.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol 39, No. 4 (August 1988), Vol 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions, incorporated herein by reference. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
Generally, in the thermal ink-jet field, an ink-jet pen or print cartridge is provided with a printhead, having an orifice plate constructed in combination with heating elements. Thermal excitation of ink near nozzles at the orifice plate is used to eject ink droplets through the miniature nozzles and orifices onto a print medium, rendering alphanumeric characters or forming graphical images using dot matrix manipulation. Other types of ink droplet generators, such as the use of piezoelectric transducers, are also known in the art. This technology is also referred to a xe2x80x9cpixel-arrayxe2x80x9d printing; the term refers to a relatively large two-dimensional imposed array or matrix of uniformly spaced and sized cells called xe2x80x9cpicture elements,xe2x80x9d or xe2x80x9cpixelsxe2x80x9d for short. By xe2x80x9cturning onxe2x80x9d certain pixels with ink, light, or the like, an image of text and graphics can be formed on the array. The intrinsic binary nature of this image becomes less obvious and the perceived image quality improves as the number of pixels per unit area increases (from unaided visual perception of individual dots at low resolutions to continuous image perception at high resolutions such as in photo-quality printing).
FIGS. 1 and 2 depict ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operations are administrated by an electronic controller 102 (usually a microprocessor-controlled printed circuit board) connected by appropriate cabling to a computer (not shown).
Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanismxe2x80x94illustrated schematically in FIG. 2xe2x80x94to an internal printing station where graphical images or alphanumeric text is created. In an exemplary media transport as shown in FIG. 2, a sheet pick device 201 delivers a sheet 105 to a transport drum 203 and pinch roller 205 nip. The sheet 105 follows the drum 203 and paper guide 204 to the printing zone 207. Looking back to FIG. 1 also, a carriage 109, mounted on a slider 111, scans the print medium in the printing zone 207. An encoder 113 is provided for keeping track of the position of the carriage 109 at any given time. A set 115 of ink-jet pens 117IN (where I=ink color, N=redundant colorant pen number), having multiple printheads firing identical ink and one black ink pen 117K, is releasably mounted in the carriage 109 for easy access. In pen-type hard copy apparatus, separate, replaceable or refillable, ink reservoirs (not shown) are located within the housing 103 and appropriately coupled to the pen set 115 via ink conduits (not shown). Once a printed page is completed, the print medium is ejected by a selectively driven star wheel 209 (FIG. 2 only) into an output tray 119. The media advance axis is defined as the y-axis, the printhead scanning axis is the x-axis, and the printhead drop firing axis is the z-axis.
For convenience of description, the word xe2x80x9cpaperxe2x80x9d will be used as synonymous for all types of print media; the word xe2x80x9cinkxe2x80x9d will be used for all compositions of colorants; the word xe2x80x9cprinterxe2x80x9d will be used for all types of hard copy apparatus. No limitation on the scope of the invention is intended nor should any be implied.
The art and technology of ink drop placement are generally referred to as xe2x80x9cprint modes.xe2x80x9d Improving print quality by placing multiple drops on each pixel or overlapped in adjoining pixels are known ink-jet printing techniques; see e.g., U.S. Pat. No. 4,963,882 filed in December 1988 by Hickman for PRINTING OF PIXEL LOCATIONS BY AN INK JET PRINTER USING MULTIPLE NOZZLES FOR EACH PIXEL OR PIXEL ROW (Hickman ""882), and U.S. Pat. No. 5,583,550 first filed in September 1989 by Hickman for INK DROP PLACEMENT FOR IMPROVED IMAGING. Hickman ""882 describes the use of using multiple nozzles per pixel location or per pixel row; this also was also known as the dot-on-dot, DOD, print mode. U.S. Pat. No. 4,999,646 filed in November 1989 by Trask for a METHOD FOR ENHANCING THE UNIFORMITY AND CONSISTENCY OF DOT FORMATION PRODUCED BY COLOR INK JET PRINTING describes a print mode of overlapping complementary dot patterns, called xe2x80x9cshingling.xe2x80x9d (Each is assigned to the common assignee herein and incorporated by reference.)
Multi-pass print modes are used to improve print quality by scanning each printed swath a number of times; see e.g., U.S. Pat. No. 4,967,203 filed in September 1989 by Doan et al. for an INTERLACE PRINTING PROCESS (assigned to the common assignee herein and incorporated by reference). In July 1989, Hickman filed for a now issued patent regarding PRINT QUALITY OF DOT PRINTERS, U.S. Pat. No. 4,965,593 (Hickman ""593). No pixel locations adjacent to each other are printed on the same traverse by a printhead. In a single printhead having at least two colorant sources, the spacing between adjacent sources in the media advance direction is made an integer (greater than one) multiple of the fixed pixel spacing. The printhead traverses the paper in a direction perpendicular to the paper advance direction, simultaneously depositing droplets of the colorant such that colorant is not deposited onto transversely adjacent pixels by the colorant sources and achieving a higher print resolution than the nozzle spacing. Advancing a paper transport stepper motor in small increments is also discussed in Hickman ""593.
In more recent ink-jet apparatus, separate printheads per color ink also have been used, mainly to improve throughput. In assignee""s co-pending patent app. U.S. patent application Ser. No. 09/311,919, D. Pinkemell shows redundant pen sets mounted in the y-axis to allow simultaneous printing of multiple swaths. Multiple like-colorant printheads per swath have also been proposed, such as in the present applicant""s U.S. patent application Ser. No. 09/233,575 for a DRUM-BASED PRINTER USING MULTIPLE PENS PER COLOR (also assigned to the common assignee here and incorporated by reference). In the basics, ink-jet pens are used in a printer so that the swaths printed by individual pens are combined into a resultant swath wider in the paper path advance axis than single pens of each ink could produce, increasing throughput. The print medium is carried on a drum and advanced through the printer. Sets of two pens, each set having the same color of ink, are carried near the drum with the two pens arranged such that the swath of one pen is adjacent to the swath of the other pen in a direction that is parallel to the drum axis. A carriage assembly provides an arrangement for combining the swath widths of the individual pens. The components of the carriage assembly are such that two pens of the same color ink are precisely positioned relative to each other, thereby to meet a very close tolerance requirement for arranging two pens of the same colorant.
Given the commercial desire for very high print resolutions, e.g., 1200+ dots-per-inch, and fast throughput, a fundamental issue of this technique is how to get adequate drop placement between drops from a first and a second (or xe2x80x9cnth) printhead of the set when the pens have intrinsic mechanical tolerance limitations of the carriage assemblies. Prior art solutions include mechanical alignment schemesxe2x80x94e.g., precision alignment boss designs, micro-machining of parts, post-assembly micro-alignment procedures. Such solutions are generally costly, complex, factory procedures and do not account for subsequent changes in mechanical alignment due to handling or due to operating conditions such as temperature change or materials creep.
Another methodology for printhead alignment improvement is to increase the spatial packing density of nozzles in each printhead array. If a perfect detection system were available, it would be possible to instruct the controller as to the real-time positional relationship of each nozzle; the closest nozzle to the correct printing target position can then be fired. Since semiconductor thin film fabrication techniques are already used to produce state of the art printheads, and nozzle sizes are already very smallxe2x80x94e.g. {fraction (1/300)}th inch diameter xe2x80x94improvements in increasing nozzle packing density are difficult, incremental in scope, and costly. A universal solution of merely increasing nozzle density does not appear to be feasible or at least commercially cost effective in the state of the art.
Another technique, shown in U.S. Pat. No. 4,621,273 by Anderson (assigned to the common assignee of the present invention and incorporated herein by reference) for a PRINT HEAD FOR PRINTING OR VECTOR PLOTTING WITH A MULTIPLICITY OF LINE WIDTHS, varies the arrangement of drop generators of the printhead. Such systems provide good results for specific image printing problems, but are not a universal fix.
Another technique, shown in U.S. Pat. No. 5,469,198 by Kadonaga (assigned to the common assignee of the present invention and incorporated herein by reference) for MULTIPLE PASS PRINTING FOR ACHIEVING INCREASED PRINT RESOLUTION has two, offset, black ink printheads on the carriage (as shown in FIG. 5 thereof) for a high quality mode, interstitial row printing in order to get 600 dot-per-inch (xe2x80x9cDPIxe2x80x9d) resolution printing in the media advance axis from 300 DPI pens. In a first pass, both pens address odd-numbered 600 DPI raster rows and, in a second pass, addressing even-numbered rows (see FIG. 22). The pens are precisely mounted in accordance with details of the disclosure therein.
In hard copy apparatus providing multiple printheads of the same colorant, there is still a need for a method and apparatus for improving ink-jet drop placement accuracy while still using simple, cost-effective printhead designs.
In its basic aspects, the present invention provides a method for placing ink drops from a plurality of scanning ink-jet printheads onto a print medium in an ink-jet hard copy apparatus, wherein the print medium is transported along a media advance axis perpendicular to a printhead scanning axis, the printheads mounted for scanning the medium along a scanning axis and each printhead having a plurality of ink drop firing nozzles arranged as at least one column of nozzles parallel to the print medium advance axis having a predetermined nozzle packing density, a known relative alignment error between printheads, and a known nozzle spacing, and the print medium having a printing surface defined as a matrix of pixels arranged as adjacent horizontal rows and vertical columns at a resolution in the media advance axis greater than the nozzle packing density, the apparatus having a means for tracking real-time position of the printheads during scanning. The method includes the steps of:
a) providing the plurality of printheads wherein at least two printheads are provided for each colorant selectively simultaneous addressing both odd and even print rows and wherein the pen-to-pen spacing is not required as an integer multiple of nozzle spacing distance;
b) during a first scan of the printheads across the print medium wherein the nozzles have a real-time known positional relationship to the matrix, scan printing a first swath of columns of dots of each colorant in rows of the matrix by firing ink drop nozzles at target pixels using printhead nozzles of each of the at least two printheads of a same colorant wherein nozzles fired for each row are logically selected with respect to the known relative alignment error;
c) advancing the medium in the print medium advance axis a distance in accordance with the equation
xe2x80x83d=(m*S)+S/n,
where
d=microstep advance distance, less than or equal to the nozzle overlap distance between printheads,
m=a value of zero or any integer,
S=nozzle spacing,
n=an integer greater than one;
d) determining a new positional relationship of the nozzles to,he matrix;
e) during a second scan of the printheads across the print medium, scan printing the swath of columns of dots of each colorant in rows of the matrix by firing ink drop nozzles at target pixels using printhead nozzles of each of the at least two printheads of a same colorant wherein nozzles in the new positional relationship fired for each row are logically selected with respect to the known relative alignment error; and
f) repeating the advancing the medium in the print medium advance axis a distance according to the equation in step c) between each scan printing of the swath until each horizontal row of target pixels has been addressed at least once.
In another basic aspect, the present invention provides an ink-jet printing method for printing a set of data with an inkjet hard copy apparatus having a plurality of ink-jet writing instruments wherein more than one instrument per colorant is mounted for scanning across a sheet of print media positioned by a transport means for selectively advancing the sheet along a print media advance axis in incremental steps through a printing zone of the apparatus, wherein each of the instruments has a plurality of nozzles arrayed in at least one column having nozzle spacing xe2x80x9cSxe2x80x9d and having a nozzle array axis parallel to the media advance axis wherein the nozzles can selectively fire ink drops onto the medium as a matrix of dotted pixels arranged as adjacent horizontal rows and vertical columns of pixels as the instruments are scanned across the sheet and wherein the instruments are mounted such that the more than one instrument per colorant will deposit ink drops in adjacent row sets of a predetermined swath of columns of pixels of the matrix, the nozzle array of each instrument of a colorant having a predetermined alignment offset to other instruments of the same colorant, the apparatus having a plurality of print mode settings for printing a range of dot resolutions on the sheet. The method includes the steps of: receiving a set of data representing a print job; selecting one of the print mode settings; setting a transport means paper advance distance as a function of the print mode setting such that the paper advance distance is a distance determined in accordance with the equation
d=(m*S)+S/n,
where d=microstep advance distance, less than or equal to the nozzle overlap distance between printheads,
m=a value of zero or any integer,
S=nozzle spacing,
n=an integer greater than one;
selecting a first data set representative of a first swath set of the set of data; performing a first scan of the writing instruments while printing data from the set representative of a first swath wherein nozzles firing drops of colorant onto all selected rows of the matrix are selected as a function of substantially instantaneous positional relationship of nozzles, including the predetermined alignment offset, to the data being printed during the scan, and wherein each colorant is selectively simultaneous addressing both odd and even print rows and wherein the pen-to-pen spacing is not required as an integer multiple of nozzle spacing distance; advancing the sheet the paper advance distance; performing another scan of the writing instruments while printing data from the set representative of the first swath wherein nozzles firing drops of colorant onto the matrix are selected as a function of substantially instantaneous positional relationship of nozzles, including the predetermined alignment offset, to the data being printed during the scan, and repeating the steps of performing another scan and advancing the sheet until the print data for from the set representative of the first swath is completely printed; selecting a next data set representative of a next swath set of the set of data and repeating the steps as for the first data set until all of the set of data has been printed.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus for printing on sheet media, the apparatus having a transport means for moving a sheet from an input along a media advance axis through a printing zone of the apparatus. The apparatus includes: a set of ink-jet pens, including at least two pens for each color ink mounted for scanning in a scan axis perpendicular to the media advance axis and including at least one column of nozzles parallel to the media advance axis for depositing ink drops as dots on a rectilinear matrix of target pixels on the sheet that is greater than nozzle packing density of the pens and can be defined by a digital print job data set and wherein the column of nozzles of each respective pen depositing ink drops of a like color ink are aligned for printing individual rows of the matrix wherein a printed swath has a greater dimension in the media advance axis than possible by a single pen of one color ink and wherein any misalignment of nozzles are determinable in a known manner; means for selecting printing resolution for the print job data set; means for setting a media advance distance at d=(m*S)+S/n, where d=microstep advance distance, less than or equal to the nozzle overlap distance between printheads, m=a value of zero or any integer, S=individual nozzle spacing, n=an integer greater than one; and means for printing the print job data set as a series of contiguous swaths of data wherein each swath is printed in multiple scans such that each colorant selectively simultaneous is addressing both odd and even print rows and wherein the pen-to-pen spacing is not required as an integer multiple of nozzle spacing distance, and the sheet is advances by the media advance distance between each scan such that printing resolution is greater than nozzle packing density.
In yet another basic aspect, the present invention provides a computer memory for an ink-jet printer, including: computer readable code means for correlating predetermined print quality characteristics, ink-jet nozzle firing algorithm routines, and predetermined multi-printhead per colorant misalignments; computer readable code means for determining a print media microstepping distance along a print media transport axis perpendicular to an ink-jet nozzle scanning axis wherein the microstepping distance is a predetermined function of nozzle spacing, up to a distance less than or equal to ink-jet nozzle overlap distance between printheads of a same ink, and the predetermined print quality characteristics; and computer readable code means for multiple scan printing of a data set representative of a print job with the printer by printing each swath of the data set printing all raster rows in each pass and using the microstepping distance for moving the print media along the transport axis between each current swath scan.
In a further basic aspect, the present invention provides an ink-jet printing device including: means for correlating predetermined print quality characteristics, ink-jet nozzle firing algorithm routines, and predetermined multi-printhead per colorant misalignments; means for determining a print media microstepping distance along a print media transport axis perpendicular to an ink-jet nozzle scanning axis wherein the microstepping distance is a predetermined function of nozzle spacing, up to a distance less than or equal to printhead nozzle overlap distance between printheads of a same ink, and the predetermined print quality characteristics; and means for multiple scan printing of a data set representative of a print job with the printer by printing each swath of the data set by printing all raster rows in each pass and using the microstepping distance for moving the print media along the transport axis between each current swath scan.
One predetermined function is expressed as: d=(m*S)+S/n, where d=microstep advance distance, less than or equal to the nozzle overlap distance between printheads, m=a value of zero or any integer, S=individual nozzle spacing, and n=an integer greater than one.
Some advantages of the present invention are:
it allows the use of existing technology, lower nozzle packing density printheads in pens having multiple printheads per colorant to achieve improved print quality in multi-pass print modes;
it provides the ability for different printheads of the same colorant to address pixels of different raster rows in high resolution in a single pass;
it enables multi-pen, high resolution addressing in a system without complex mechanical devices to resolve pen alignment problems;
it provides for a lower cost of manufacture;
it provides higher addressable resolution in the paper transit axis than the inherent nozzle packing density; and
it provides improved print quality.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.